Pervaporation sorption

Process Description Pervaporation is a separation process in which a liquid mixture contacts a nonporous permselective membrane. One component is transported through the membrane preferentially. It evaporates on the downstream side of the membrane leaving as a vapor. The name is a contraction of permeation and evaporation. Permeation is induced by lowering partial pressure of the permeating component, usually by vacuum or occasionally with a sweep gas. The permeate is then condensed or recovered. Thus, three steps are necessary Sorption of the permeating components into the membrane, diffusive transport across the nonporous membrane, then desorption into the permeate space, with a heat effect. Pervaporation membranes are chosen for high selectivity, and the permeate is often highly purified. 

The main emphasis in this chapter is on the use of membranes for separations in liquid systems. As discussed by Koros and Chern(30) and Kesting and Fritzsche(31), gas mixtures may also be separated by membranes and both porous and non-porous membranes may be used. In the former case, Knudsen flow can result in separation, though the effect is relatively small. Much better separation is achieved with non-porous polymer membranes where the transport mechanism is based on sorption and diffusion. As for reverse osmosis and pervaporation, the transport equations for gas permeation through dense polymer membranes are based on Fick s Law, material transport being a function of the partial pressure difference across the membrane. 

From Fig. 19.3a-c, and as opposed to purely sorption controlled processes, it can be seen that during pervaporation both sorption and diffusion control the process performance because the membrane is a transport barrier. As a consequence, the flux 7i of solute i across the membrane is expressed as the product of both the sorption (partition) coefficient S, and the membrane diffusion coefficient Di, the so-called membrane permeability U, divided by the membrane thickness f and times the driving force, which maybe expressed as a gradient of partial pressures in place of chemical potentials [6] ... 

In comparison with adsorptive/absorptive techniques for aroma recovery from bioconversions, the disadvantage of pervaporation is the fact that both sorption and diffusion determine the overall selectivity. While the sorption selectivity is very high (equal to that of adsorptive/absorption), the diffusion selectivity favours water owing to the simple fact that water is a smaller molecule than aroma compounds and thus sterically less hindered during diffusion (Table 19.1). The overall (perm)selectivity P=SD) is therefore lower than in strictly sorption controlled processes, although it is still favourable compared with that for evaporation. This shortcoming compares, however, with operational advantages of pervaporation as outlined before. 

U.S. Toti, M.Y. Kariduraganavar, K.S. Soppimath, T.M. Aminabhavi, Sorption, diffusion and pervaporation separation of water/acetic acid mixtures through the blend membranes of sodium alginate and guar gum-g-poly(acrylamide), J. Appl. Polym. Sci. 83 (2002) 259-272. 

Sorption data were used to obtain values for A" L. As pointed out by Paul and Paciotti, the data in Figure 2.17 show that reverse osmosis and pervaporation obey one unique transport equation—Fick s law. In other words, transport follows the solution-diffusion model. The slope of the curve decreases at the higher concentration differences, that is, at smaller values for c,eimi because of decreases in the diffusion coefficient, as the swelling of the membrane decreases. 

The selectivity (amcm) of pervaporation membranes critically affects the overall separation obtained and depends on the membrane material. Therefore, membrane materials are tailored for particular separation problems. As with other solution-diffusion membranes, the permeability of a component is the product of the membrane sorption coefficient and the diffusion coefficient (mobility). The membrane selectivity term amem in Equation (9.11) can be written as... 

Pervaporation is a concentration-driven membrane process for liquid feeds. It is based on selective sorption of feed compounds into the membrane phase, as a result of differences in membrane-solvent compatibility, often referred to as solubility in the membrane matrix. The concentration difference (or, in fact, the difference in chemical potential) is obtained by applying a vacuum at the permeate side, so that transport through the membrane matrix occurs by diffusion in a transition from liquid to vapor conditions (Figure 3.1). Alternatively, a sweep gas can be used to obtain low vapor pressures at the permeate side with the same effect of a chemical potential gradient. 

Selective separation of hquids by pervaporation is a result of selective sorption and diffusion of a component through the membrane. PV process differs from other membrane processes in the fact that there is a phase change of the permeating molecules on the downstream face of the membrane. PV mechanism can be described by the solution-diffusion mechanism proposed by Binning et al. [3]. According to this model, selective sorption of the component of a hquid mixture takes place at the upstream face of the membrane followed by diffusion through the membrane and desorption on the permeate side. 

Samdani AR, Mandal S, and Pangarkar VG. Role of and criterion for sorption selectivity in pervaporative removal of trace organics from aqueous solutions. Sep. Sci. Tech. 2003 38(5) 1069-1092. 

Sferrazza RA, Escobosa R, and Gooding CH. Estimation of parameters in a sorption-diffusion model of pervaporation. J. Memb. Sci. 1988 35(2) 125-136. 

The polymer materials mainly used for the membranes are glassy polymers, the first and foremost polyimides. The use of glassy polymers having a rigid ensemble of macromolecules results in high separation effectiveness. Separation effectiveness in pervaporation processes is characterized by the separation factor, /3p, which is determined by the diffusion component, /3d, and the sorption component, /3s [8,55]. Let us consider the effect of chemical composition of polymer membranes on their transport properties with respect to aromatic, alicyclic, aliphatic hydrocarbons and analyze ways to improve these properties. 

To increase the sorption component of the separation factor, homogeneously distributed tetracyanoethylene, a strong electron acceptor having high affinity for electron donors, was added to the polyimide matrix [77]. It can be seen from data presented in Table 9.12 that this is accompanied by an increase in the sorption component /3s (benzene/cyclohexane) by a factor of 1.5 probably as a result of selective sorption of aromatic compounds by tetracyanoethylene with a simultaneous increase in the diffusion component /3d. The prepared membranes showed good pervaporation properties with respect to benzene/cyclohexane, toluene/isooctane mixtures. For example, for a two-component 50/50 wt% benzene/cyclohexane mixture at 343 K, the flux was 2 = 0.44 kg p,m/m h, and /3p (benzene/cyclohexane) = 48 and for a two-component toluene/isooctane mixture, 45/55 wt%, at 343 K the flux was 2 = 1-1 kg p-m/m h, and /3p (toluene/wo-octane) = 330. 

Carboxylic Groups Pervaporation separation of toluene/i-octane mixmres using copolyimide membranes containing 3,5-diaminobenzoic acid (DABA) was investigated in Ref. [128]. It was established that introduction of diaminobenzoic acid into the 6FDA-TrMPD polyimide improves membrane selectivity. The sorption component of the separation factor /3s is hnearly correlated with the membrane solubility parameter and with DABA content in the copolymer (/3s = 3.2, 3.3,4.3, 5.2 for DABA contents 0%, 10%, 33%, 60%, respectively). 

Sun F and Ruckenstein E. Sorption and pervaporation of benzene-cyclohexane mixtures through composite membranes prepared via concentrated emulsion polymerization. J Membr Sci 1995 99 273-284. 

Kao ST, Wang FG, and Lue SJ. Sorption, diffusion and pervaporation of benzene/cyclohexane mixtures on silver-Nafion membranes. Desalination 2002 149 35-40. 

Larcbet C, Bmn JP, Bulvestre G, and Auclair B. Sorption and pervaporation of dilute aqueous solutions of organic compounds through polymer membranes. J Membr Sci 1985 25 55-100. 

The mechanism of transport by pervaporation can be described in the light of the sample diffusion model [159], which comprises the following steps (a) evaporation of the analyte into the air gap (b) sorption into the membrane on the sample side (c) diffusion of the sorbed component through the polymer matrix and (d) desorption into a liquid or gas phase on the acceptor side. The last three steps are also included in industrial pervaporation processes. 

Sorption and diffusion in polymers are of fundamental and practical concern. However, data acquisition by conventional methods is difficult and time consuming. Again, IGC represents an attractive alternative. Shiyao and co-workers, concerned with pervaporation processes, use IGC to study adsorption phenomena of single gases and binary mixtures of organic vapors on cellulosic and polyethersulfone membrane materials (13). Their work also notes certain limitations to IGC, which currently restrict its breadth of application. Notable is the upper limit to gas inlet pressure, currently in the vicinity of 100 kPa. Raising this limit would be beneficial to the pertinent use of IGC as an indicator of membrane-vapor interactions under conditions realistic for membrane separation processes. 

Pervaporation is a separation technique that has several advantages over other separation methods such as distillation, extraction, and sorption. It is an especially good process for certain azeotropes and mixtures that have components with similar boiling points. Also, pervaporation can be performed at low temperatures and the membranes involved generally do not need to be regenerated in any way.[161]... 

Hauser, J. Heintz, A. Reinhard, G.A. Schmittecker, B. Wesslein, M. Lichtenthaler, R.N. Sorption, diffusion, and pervaporation of water—alcohol mixtures in PVA-membranes. Proceedings of the 2nd International Conference on Pervaporation... 

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